Extension device for a bone anchor, in particular for minimally invasive surgery

ABSTRACT

An extension device for a bone anchor is provided, wherein the bone anchor comprises an anchoring section and a receiving part connected to the anchoring section, the receiving part having a central axis, the extension device comprising a first sleeve with a first sleeve axis that is coaxial to the central axis and being configured to be coupled to and decoupled from the receiving part; a second sleeve with a second sleeve axis coaxial to the central axis, being positioned within the first sleeve and positionable relative to the first sleeve along the central axis, and being configured to be coupled to and decoupled from the receiving part; a locking member that, in a first configuration, inhibits decoupling of the first sleeve or the second sleeve from the receiving part when the other one of the first sleeve or the second sleeve is coupled to the receiving part.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present disclosure claims the benefit of U.S. Provisional Patent Application Ser. No. 62/013,421, filed on Jun. 17, 2014, the contents of which are hereby incorporated by reference in their entirety, and claims priority to European Patent Application EP 14172835.2, filed on Jun. 17, 2014, the contents of which are hereby incorporated by reference in their entirety.

BACKGROUND

1. Field of the Invention

The invention relates to an extension device for a bone anchor, in particular for use in minimally invasive surgery (MIS). The invention also relates to a system including such an extension device and a bone anchor, wherein the bone anchor comprises an anchoring section and a receiving part for receiving a rod to couple the rod to the anchoring section. The extension device includes a first sleeve and a second sleeve that are each configured to be coupled to the receiving part in such a manner that a translational and a rotational movement of the extension device relative to the receiving part is inhibited. A locking member is provided that guarantees correct decoupling of the sleeves from the receiving part.

2. Description of the Related Art

Extension devices, also called head extenders, for pedicle screws for use in minimally invasive surgery are known in the art. For example, U.S. Pat. No. 7,563,264 B2 describes a spinal stabilization system for a minimally invasive procedure wherein detachable sleeves may be coupled to a collar of a bone anchor to allow for formation of the spinal stabilization system through a small skin incision. The detachable sleeve members may allow for alignment of the collars to facilitate insertion of an elongated member in the collars. A coupling system is provided between the sleeve and the collar that inhibits translational movement of the sleeve relative to the collar. In one embodiment, the sleeve may be coupled to a collar of a bone fastener assembly with movable members that may be threaded into threaded openings in the collar.

WO 2013/112689 A2 describes a minimally invasive tower access device comprising an elongated outer sleeve that slidably receives an elongated inner sleeve. A lock nut is used to secure the inner sleeve and outer sleeve in a locked mode.

There is still a need for an extension device for a bone anchor that is not only safe during use in the surgical steps, such as compression and distraction, but that is also safe during the step of decoupling from the receiving part.

SUMMARY

It is the object of the invention to provide an improved extension device for a bone anchor, in particular for use in minimally invasive surgery, and a system of such an extension device and a bone anchor that facilitates the surgical steps and improves the safety of the device.

The extension device is configured to be coupled to the receiving part such that it is locked against translational and rotational movement of the extension device relative to the receiving part. Because translational and rotational movements of the extension device and the receiving part are inhibited, the connection between the extension device and the receiving part is robust. This permits a safe placement of the rod and a set screw for fixing the rod. Surgical steps of adjustment of the spinal stabilization system, such as compression or distraction can be performed using the extension device attached to the receiving part, once the bone anchor has been inserted into the bone. In one embodiment, the coupling between the extension device and the receiving part is effected by a formfit engagement of a portion of the extension device with a portion of the receiving part. In another embodiment, the coupling is effected by a frictional engagement of a portion of the extension device with a portion of the receiving part. The coupling can also be partially a form-fit coupling and partially a friction-fit coupling.

The extension device comprises a first sleeve and a second sleeve positioned within the first sleeve and an interlocking bushing that connects the second sleeve to the first sleeve and allows a controlled axial movement of the second sleeve relative to the first sleeve. After the extension device has been attached to the receiving part, the first sleeve and the second sleeve can be interlocked with each other and with the receiving part by operating the interlocking bushing in one direction. The interlocking connection between the first sleeve and the second sleeve can be released by operating the interlocking bushing in the opposite direction.

In one embodiment, a translational movement between the extension device and the receiving part can be inhibited by a form-fit engagement of a circumferential rib that extends at least partially around the longitudinal axis of the device and engages a corresponding circumferential groove. The rib may be provided at the first sleeve and the groove at the receiving part or vice versa.

In another embodiment, the first sleeve may be coupled to the receiving part by means of a form-fit connection and the second sleeve may be coupled to the receiving part by a friction-fit conection only or vice versa

Decoupling of the extension device from the receiving part can be carried out only in a prescribed sequence of steps which in one embodiment consists in a first step of decoupling the second sleeve and a second step of decoupling the first sleeve. The correct sequence of steps is guaranteed by the presence of a locking member that, in a first configuration, inhibits the decoupling of the first sleeve when the second sleeve is still coupled to the receiving part. When the locking member is in a second configuration after decoupling the second sleeve, the decoupling of the first sleeve is allowed. Hence, a damage of the extension device based on erroneous operation is prevented. Moreover, with the use of such an extension device the surgeon can perform a next step in the surgical procedure only after correct decoupling of the extension device. This enhances the safety of the procedure.

The extension device may comprise a third sleeve that can be removably connected to the first sleeve in order to provide an extension device having an increased length. For example, a surgical procedure may be started with the longer extension device that comprises the third sleeve attached to the first sleeve for placement of the rod and the set screw. Thereafter, the third sleeve may be removed in order to improve the placement of the receiving part relative to the rod during compression and distraction steps. With the shorter extension device a greater variety of angles of the receiving parts relative to each other may be obtained.

The extension device comprises only few parts which facilitate the assembly and operation of the device.

The extension device may be used together with a release or decoupling instrument that is adapted to engage the extension device. The locking member of the extension device is configured to be engaged by the decoupling instrument in its second configuration only. It cannot be engaged in the first configuration in which both, the first and the second sleeve are coupled to the receiving part. Hence, the system of the extension device and the decoupling instrument guarantees the safety of handling and prevents damage of parts of the extension device or the receiving part.

As the spinal stabilization system includes at least two bone anchors with receiving parts that are adapted to be used with the extension device.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparent from the description of embodiments using the accompanying drawings. In the drawings:

FIG. 1 shows a perspective exploded view of the extension device according to a first embodiment.

FIG. 2 shows a perspective partially exploded view of the extension device of FIG. 1 in an assembled state with an optional third sleeve.

FIG. 3 shows a perspective view of the extension device of FIG. 2 in an assembled state.

FIG. 4 shows a cross-sectional view of an upper portion of the extension device of FIGS. 2 and 3 with the attached third sleeve, wherein the cross-section is taken in a plane including the sleeve axis.

FIG. 5 shows a perspective view of an embodiment of a receiving part of a bone anchor that may form together with the extension device of FIGS. 1 to 4 a first embodiment of a system of an extension device and a bone anchor.

FIG. 6 shows a top view of the receiving part of FIG. 5.

FIG. 7 a shows a cross-sectional view of the receiving part of FIG. 5 and FIG. 6 along the lines A-A′ in FIG. 6.

FIG. 7 b shows an enlarged view of a detail of FIG. 7 a.

FIG. 8 a shows a perspective view of a first sleeve of the extension device of FIGS. 1 to 4.

FIG. 8 b shows a perspective view from the bottom of a front end portion of the first sleeve of FIG. 8 a.

FIG. 9 shows a top view of the first sleeve of FIG. 8 a.

FIG. 10 shows a cross-sectional view of the first sleeve of FIG. 8 a along line B-B in FIG. 9.

FIG. 11 shows a cross-sectional view of the first sleeve of FIG. 8 a along line D-D in FIG. 10.

FIG. 12 a shows a perspective view of a second sleeve of the extension device of FIGS. 1 to 4.

FIG. 12 b shows an enlarged perspective view from the bottom of a front end portion of the second sleeve of FIG. 12 a.

FIG. 13 shows a cross-sectional view of the second sleeve according to FIG. 12 a, the cross-section taken in a plane containing the sleeve axis and extending through the center of the legs of the sleeve.

FIG. 14 shows an exploded perspective view of an interlocking bushing of the extension device of FIGS. 1 and 2 and an end portion of the second sleeve.

FIG. 15 shows a perspective view of the interlocking bushing and the end portion of the second sleeve of FIG. 14 in an assembled state.

FIG. 16 shows a top view of the interlocking bushing of FIG. 14.

FIG. 17 shows a cross-sectional view of the interlocking bushing of FIG. 14 along line E-E in FIG. 16.

FIG. 18 shows a perspective view of a bone anchor with a receiving part and a front portion of the extension device according to the first embodiment coupled to the receiving part.

FIG. 19 shows a cross-sectional view of an upper portion of the extension device, without third sleeve, and a front end portion of a decoupling instrument to be used with and adapted to the extension device, the cross-section taken in a plane including the sleeve axis.

FIG. 20 shows a perspective exploded view of the third sleeve.

FIG. 21 shows a top view of the third sleeve shown in FIG. 20.

FIG. 22 shows a cross-sectional view of the third sleeve of FIGS. 20 and 21 along line F-F in FIG. 21.

FIG. 23 a shows a cross-sectional view of a first step of coupling the extension device to the receiving part of a bone anchor.

FIG. 23 b shows a cross-sectional view of an upper portion of the extension device in the first step according to FIG. 23 a, wherein the interlocking bushing is in a second configuration.

FIG. 24 a shows a cross-sectional view of a second step of coupling the extension device to the receiving part of a bone anchor.

FIG. 24 b shows a cross-sectional view of an upper portion of the extension device in the step of FIG. 24 a, wherein the interlocking bushing is still in the second configuration.

FIG. 25 a shows a cross-sectional view of a third step of coupling the extension device to the receiving part of a bone anchor.

FIG. 25 b shows a cross-sectional view of an upper portion of the extension device in the step of FIG. 25 a, wherein the interlocking bushing is in a first configuration.

FIG. 26 shows a perspective view of a system comprising at least two bone anchors with a receiving part and extension devices with different length coupled thereto.

DETAILED DESCRIPTION

As shown in FIGS. 1 to 3, an extension device according to a first embodiment includes a first sleeve 1 that forms an outer sleeve, a second sleeve 2 that forms an inner sleeve and that is positionable within the first sleeve 1 and an interlocking bushing 3. The interlocking bushing 3 is configured to be connected to the second sleeve 2 and is configured to couple the second sleeve 2 to the first sleeve 1 such as to permit a controlled motion of the second sleeve 2 relative to the first sleeve 1. In addition, a third sleeve 4 may be included in the extension device. The third sleeve 4 can be removably attached to the first sleeve 1 to extend the length of the first sleeve 1. When the first sleeve 1 and the second sleeve 2 are assembled as shown in FIGS. 2 and 3, the extension device can be used to place the parts of a spinal stabilization system in a patient's body using a minimally invasive procedure. For some particular steps in the surgical procedure, the third sleeve 4 may be removed.

The extension device shown in FIGS. 1 to 3 is configured to be used with a bone anchor for example with a pedicle screw. As depicted in FIGS. 5 to 7 b and 23 a to 25 b, an example of such a bone anchor may comprise an anchoring element with a threaded shank 100 and a spherical segment-shaped head 101, wherein the anchoring element is pivotably coupled to a receiving part 200. The receiving part is shown in more detail in FIGS. 5 to 7 b. Typically, the receiving part may be formed as a substantially cylindrical part with a first end or top end 200 a and a second end or bottom end 200 b, a central axis C, a coaxial bore 201 extending from the top end 200 a to a distance from the bottom end 200 b, and a seat 202 for the head 101 of the anchoring element as well as a lower opening 203 at the bottom end 200 b where the shank of the bone anchoring element can pass through. A substantially U-shaped recess 204 may extend from the top end 200 a in the direction of the bottom end 200 b. The recess 204 serves for receiving a rod (not shown) therein. By means of the recess 204 two free legs 205 a, 205 b are formed. At an outer surface of the legs 205 a, 205 b at a distance from the top end 200 a, circumferentially extending grooves 206 a, 206 b are formed that extend from one end of the channel formed by the U-shaped recess 204 to the other end and that are open towards the U-shaped recess 204. An upper sidewall 206 a′, 206 b′ and a lower sidewall 206 a″, 206 b″ of the groove 206 a, 206 b may be inclined towards the bottom end 200 b.

The receiving part 200 may further include at the top end 200 a in a free end surface of each of the legs 205 a, 205 b a recess 207 a, 207 b that extends into the legs in a direction parallel to the central axis C. The recess 207 a, 207 b is in a top view elongate and closed in a circumferential direction at both ends. Both recesses 207 a, 207 b serve for engagement with a portion of the extension device as described below. There may be a chamfered section 208 provided at the top end of each recess 207 a, 207 b that facilitates engagement of the recess 207 a, 207 b with a corresponding protrusion at the extension device.

In addition, the receiving part 200 may comprise a pair of longitudinal grooves 212 a at one side of the U-shaped recess 204 and a pair of longitudinal grooves 212 b at the opposite side. The grooves of each pair of grooves 212 a, 212 b are spaced apart from each other in the circumferential direction. Moreover, the grooves extend from the top end 200 a in the outer wall of the receiving part on either side of the recesses 207 a, 207 b and are open towards the top end 200 a. The grooves may have a decreasing depth towards their closed end that is located at a distance from the top end 200 a. The cross-section of the longitudinal grooves 212 a, 212 b is substantially rectangular. As illustrated in particular in FIG. 5, the longitudinal grooves 212 a, 212 b intersect the circumferentially extending grooves 206 a, 206 b. The grooves 212 a, 212 b may interact with corresponding longitudinally extending pairs of ribs of the first sleeve in order to provide a form-fit connection.

In an upper portion of the legs 205 a, 205 b, an internal thread 209 may be provided for cooperating with a locking screw (not shown) for fixing the rod.

Referring to FIGS. 8 a to 10, the first sleeve 1 of the extension device is elongate and comprises a longitudinal axis c that is coaxial with the central axis C of the receiving part 200 when the extension device is coupled to the receiving part. The first sleeve 1 further has a front end 1 a and a rear end 1 b.

Adjacent to the rear end 1 b, a first section 11 is provided with an inner diameter that is greater than an inner diameter of a following second section 12. Two small projections 11 a, 11 b protrude from the rear end 1 b. They may interact with recesses in the third sleeve 4 for correct positioning of the third sleeve 4 onto the first sleeve 1. The second section 12 that follows the first section 11 serves the function of providing a clamping portion for the interlocking bushing 3 as described below. Adjacent to the second section 12, a third section 13 follows that may have a slightly smaller inner diameter compared to the second section 12. In the third section 13, an internal thread 15 is provided that is configured to interact with the interlocking bushing 3. The internally threaded section 15 is preferably adjacent to the second section 12. The first sleeve I may be in a portion thereof at 90° offset from the projections 11 a, 11 b double-walled by means of arc-shaped or cylinder segment-shaped longitudinal recesses 19 c, 19 d.

The first sleeve 1 may further comprise in its outer wall at a first distance from the rear end 1 b two opposite first transverse recesses 15 a, 15 b that serve for attaching an instrument to the first sleeve 1. The recesses 15 a, 15 b may be at circumferential positions corresponding to the small projections 11 a, 11 b. Moreover, at a second distance from the rear end 1 b two second recesses 16 a, 16 b may be provided, that extend in a circumferential direction and that are substantially 90° offset from the recesses 15 a, 15 b. The second transverse recesses 16 a, 16 b extend fully through an outermost wall of the first sleeve 1 and may serve for cleaning purposes to provide access to the recesses 19 c, 19 d.

Two elongate substantially U-shaped slits 17 a, 17 b that have a reverse U-shape compared to the slit 204 in the receiving part and that are offset from each other by 180° extend from the front end 1 a towards the rear end 1 b up to a distance from the second section 12 of the first sleeve 1. The longitudinal slits 17 a, 17 b have a width in a circumferential direction that is greater than a diameter of a spinal stabilization rod such that they permit to insert the rod through the slits. Preferably, the width of the slits 17 a, 17 b is substantially the same as the width of the U-shaped recess of the receiving part 200. The length of the slits 17 a, 17 b is typically more than 1/3 of the length of the first sleeve, preferably more than ½ of the length of the first sleeve. They can even be longer than ¾ of the length of the first sleeve. The slits are losated at a position that is offset by approximately 90° from the position of the first recesses 15 a, 15 b. By means of the slits 17 a, 17 b, two free legs 18 a, 18 b are formed that serve the purpose of interacting with the free legs 205 a, 205 b of the receiving part 200. By means of the legs 18 a, 18 b, the first sleeve 1 is slightly flexible in a direction perpendicular to the sleeve axis, so that the first sleeve 1 can be snapped onto the legs 205 a, 205 b of the receiving part 200.

The inner surface of the third section 13 of the first sleeve 1 comprises at each of the legs 18 a, 18 b a longitudinally extending substantially arc-shaped or cylinder segment-shaped guiding recess 19 a, 19 b into which a portion of the second sleeve 2 can extend to be guided therein. The guiding recesses 19 a, 19 b extend in a longitudinal direction beyond the slits 17 a, 17 b towards the rear end 1 b, as can be seen in particular in FIG. 10.

Each of the legs 18 a, 18 b comprises at a distance from the front end 1 a an inwardly directed projection 120 a, 120 b that extends in a circumferential direction around the longitudinal axis c from one slit 17 a to the opposite slit 17 b. The shape of the circumferential projection 120 a, 120 b is substantially complementary to the shape of the circumferential grooves 206 a, 206 b of the receiving part 200, as can be seen in particular in FIG. 18 or 24 a, 25 a.

An upper surface of the projections 120 a, 120 b that faces towards the rear end 1 b, may be inclined towards the rear end to match the complementary inclined upper wall 206 a′, 206 b′ of the groove 206 a, 206 b. A lower surface of the circumferential projections 120 a, 120 b may be perpendicular to the central axis or also inclined towards the rear end 1 b. The inclination facilitates the engagement of the projections 120 a, 120 b into the groove 206 a, 206 b when the first sleeve 1 is coupled to the receiving part 200.

In addition, the first sleeve 1 comprises at positions that correspond in a circumferential direction to the positions of the longitudinal grooves 212 a, 212 b of the receiving part 200, a pair of ribs 112 a, and a pair of ribs 112 b on the legs 18 a, 18 b. The ribs 112 a, 112 b protrude inwards from the legs 18 a, 18 b and downwards from the front end 1 a and have a shape that substantially matches the shape of the grooves 212 a, 212 b. As can be seen in particular in FIG. 8 b, an outer surface of the ribs 112 a, 112 b tapers towards the free end, such that, when seen in a circumferential direction, the cross-section of the ribs is substantially triangular.

From each of the longitudinal ribs 112 a, 112 b, a transverse thickened rib portion 123 a, 123 b extends outward in a circumferential direction. This transverse rib portion 123 a, 123 b is located at a distance from the front end 1 a that corresponds to the distance of the circumferentially extending groove 206 a, 206 b from the top end 200 a. Hence, the transverse rib portion 123 a, 123 b is at the same level as the circumferential projections 120 a, 120 b.

By means of the longitudinal ribs 112 a, 112 b that are configured to engage the longitudinal grooves 212 a, 212 b, respectively, the strength of the connection between the head extension device and the receiving part is enhanced. The longitudinal ribs and grooves allow to apply a high torque with the extension device to the receiving part.

An overall outer shape of the first sleeve 1 may be cylindrical. Flattened outer surface portion 122 a, 122 b may be provided at an outer surface of the legs 18 a, 18 b for interacting with a tool (not shown).

The total length of the first sleeve is such that when the bone anchor is inserted into the bone and the first sleeve 1 is attached to the receiving part 200, the extension device protrudes to a sufficient extent from the operation site.

As illustrated in FIGS. 12 a to 13, the second sleeve 2 comprises a front end 2 a and an opposite rear end 2 b. The second sleeve 12 has a substantially constant outer diameter. Adjacent to the rear end 2 b, the second sleeve 2 comprises a first portion 21 with a circumferentially closed cylinder surface. The first portion 21 is configured to engage the interlocking bushing 3. A plurality of longitudinal slits 22 that are open to the rear end 2 b and extend up to a distance from the rear end 2 b, are provided to render the first portion 21 flexible in such a way that the first portion 21 can elastically snap onto a portion of the interlocking bushing 3 and is held there by friction. At a first distance from the rear end 2 b an inwardly extending annular projection 23, is provided. The annular projection 23 interacts with a corresponding depression or a groove at the interlocking bushing 3 to inhibit an inadvertent removal of the interlocking bushing 3 from the second sleeve 2. At a second distance from the rear end 2 b that is greater than the first distance, a stop 21 a, for example in the form of an annular shoulder, is provided. The shoulder 21 a limits the insertion of the interlocking bushing into the second sleeve 2 and forms an abutment when the interlocking bushing is screwed downward into the first sleeve 1. An outer diameter of the first section 21 of the second sleeve 2 is smaller than an inner diameter of the third section 13 of the first sleeve 1.

Two recesses 24 a, 24 b with a substantially rectangular cross-section extend from the front end 2 a through the second sleeve 2 up to the first portion 21. The recesses 24 a, 24 b have such a size that two opposite legs 25 a, 25 b are formed that fit into the guiding recesses 19 a, 19 b of the first sleeve 1. The legs 25 a, 25 b have such a length that they extend beyond the upper closed end of the recesses 17 a, 17 b of the first sleeve 1 in a direction towards the rear end 1 b of the first sleeve 1.

The front end 2 a of the second sleeve 2 is comprised of a substantially flat surface portion 26 a, 26 b on each of the legs 25 a, 25 b. This flat surface portion 26 a, 26 b is configured to cooperate with a substantially flat surface portion on the top end 200 a of the receiving part. Moreover, a projections 27 a, 27 b on each leg that is configured to cooperate with the recess 207 a, 207 b at the free end of the legs of the receiving part 200 is provided. Hence, the projection 27 a, 27 b has a substantially complementary shape to the shape of the recess 207 a, 207 b. An inner surface of the projections 27 a, 27 b is flush with the inner surface of the legs 25 a, 25 b. An outer surface of the projections 27 a, 27 b is slightly recessed with respect to an outer surface of the legs 25 a, 25 b. The overall shape of the projections 27 a, 27 b is substantially arc-shaped with rounded edges corresponding to the recesses 207 a, 207 b in the receiving part. Moreover, in a circumferential direction, the projections 27 a, 27 b are arranged substantially in the middle of the legs 25 a, 25 b.

Turning now to FIGS. 14 to 17, the interlocking bushing 3 comprises a front end 3 a and a rear end 3 b, a cylindrical section 31 adjacent to the front end 3 a and that has an outer diameter substantially equal to an inner diameter of the first section 21 of the second sleeve 2 so that the bushing 3 is held in the second sleeve 2 by friction. Adjacent to the cylindrical section 31 with a smooth outer surface, there is a groove 32 that interacts with the annular projection 23 of the second sleeve 2. Following the groove 32 there is a threaded portion 33 with an external thread that is configured to interact with the threaded section 14 of the first sleeve 1.

The interlocking bushing 3 has an inner diameter in the threaded portion 33 that is slightly larger than D₁ wherein D₁ corresponds to an outer diameter of a decoupling instrument. A plurality of longitudinal engagement grooves 34 for a driver are provided in the inner wall of the interlocking bushing 3 at an upper region of the threaded portion 33. Adjacent to the rear end 3 b, there is a collet portion 35 that is characterized by a plurality of substantially longitudinal slits 36 that are open towards the rear end 3 b. The slits 36 render the collet portion 35 flexible, such that it can be radially compressed by exerting pressure from the outside. The collet portion 35 comprises at the rear end 3 b of the interlocking bushing a thickened annular portion 37 that has a lower side 37 a that tapers toward the sleeve axis and an upper side 37 b that inversely tapers toward the sleeve axis so that a cross-section of the thickened portion 37 is substantially triangular. The length of the collet portion 35 is such that when the interlocking bushing 3 is screwed into the first sleeve 1 and the second sleeve 2 engages the receiving part 200, most part of the thickened portion 37 of the collet portion 35 is located within the second portion 12 of the first sleeve 1, as depicted more in detail in FIG. 19. In this position, the collet portion 35 is slightly compressed by the inner wall of the first sleeve 1 so that an inner diameter D₂ of the collet portion 35 is smaller than D₁ as shown in detail in FIG. 19. When the interlocking bushing 3 is screwed backward the collet portion 35 emerges from the second portion 12 whereby it expands.

Hence, the collet portion 35 forms a locking member that can assume a first configuration in which it is compressed and inhibits the insertion of a tool that removes the first sleeve 1 from the receiving part as long as the second sleeve 2 is still engaging the receiving part and that can assume a second configuration in which it is expanded and allows the insertion of the tool when the second sleeve 2 is decoupled. In this embodiment, the locking member is monolithically formed with the interlocking bushing 3.

The assembly of the second sleeve 2 and the interlocking bushing 3 is shown in FIGS. 14 and 15. The interlocking bushing is pushed with its cylindrical portion 31 into the first section 21 of the second sleeve 2 until the annular projection 23 snaps into the groove 32. By means of this, the interlocking bushing 3 is coupled to the second sleeve 2 in such a manner, that a rotational motion of the interlocking bushing 3 relative to the second sleeve 2 is possible. An axial movement of the interlocking bushing 3 relative to the second sleeve 2 is inhibited when the interlocking bushing abuts against the stop 21 a provided in the second sleeve 2. The connection between the second sleeve 2 and the first sleeve 1 via the interlocking bushing 3 is a rigid connection.

The assembly comprising the second sleeve 2 and the interlocking bushing 3 in a mounted state as shown in FIG. 15 is then inserted into the first sleeve 1 from the rear end 1 b until the threaded portion 33 of the interlocking bushing 3 engages the threaded section 14 of the first sleeve 1. The legs 25 a, 25 b are guided in the guiding recesses 19 a, 19 b in the first sleeve.

As depicted in FIG. 19, a release or decoupling instrument 50, only a front portion of which is shown, comprises an inner first portion 51 and an outer portion 52 that slidably receives the inner portion 51. The outer portion 52 may be expandable and serves for decoupling the first sleeve 1 from the receiving part 200. It has an outer diameter D₁ so that the outer portion 52 can be guided through the interlocking bushing 3 into the first sleeve 1 and can be spread there. Thereby, the first sleeve 1 can be decoupled from the receiving part.

The decoupling instrument 50 can be inserted into the interlocking bushing 3 only, if the collet portion 35 is in the second configuration, where it protrudes outward of the second portion 12 into the first portion 11 of the first sleeve 1. Because the first portion 11 has a greater inner diameter than the second portion 12, the collet portion 35 can expand therein so that the inner diameter is sufficiently large to allow insertion of the decoupling instrument 50.

Referring to FIGS. 20 to 22, the optional third sleeve 4 will be described. The third sleeve 4 has a first or front end 4 a and a second or rear end 4 b. Its outer diameter is preferably greater than the outer diameter of the first sleeve 1. Adjacent to its rear end 4 b it comprises two opposite longitudinal slits 41 a, 41 b that extend from the second end 4 b along at least a portion of the third sleeve that may be up to ½ of the length of the third sleeve. One of the sidewalls of each longitudinal slit 41 a, 41 b comprises a wavy structure for latching with a reduction sleeve (not shown) used for further steps in the surgical procedure, for example for pressing down the rod and inserting a locking screw to fix the rod. Furthermore, an engagement structure 42, for example, a plurality of flat engagement portions, is provided at an outer surface of an upper portion of the third sleeve for applying a tool thereto.

The third sleeve 4 has a first inner portion 43 adjacent to the rear end 4 b and a second inner portion 44 adjacent to the lower end 4 a. The second inner portion 44 is configured to accommodate the upper portion of the first sleeve 1 therein. For this purpose, the inner diameter of the second section 44 is slightly larger than the outer diameter of the upper end of the first sleeve 1. An upper end of the second portion 44 may provide a shoulder 44 a, 44 b that forms an abutment for the rear end 1 b of the first sleeve. The shoulder may have an interruption at positions 45 a, 45 b that correspond to the small protrusions 11 a, 11 b of the first sleeve. An inner diameter of the first portion 43 may be larger than an inner diameter of the second portion 44.

At a distance from the lower end 4 a two transverse slots 46 a, 46 b are provided at positions that are 180° offset from each other. The slots 46 a, 46 b are configured to accommodate an operating pusher 47 therein. The slots 46 a, 46 b are elongate in a circumferential direction and preferably positioned at 90° with respect to the longitudinal slits 41 a, 41 b. Moreover, in the second section 44, there is at the bottom of one of the elongate transverse slots 46 a a substantially rectangular recess 48 that serves for accommodating an abutment plate 49 therein.

The pusher 47 is a flat piece that can extend through the slots 46 a, 46 b. It has two substantially parallel longitudinal outer walls 47 a, 47 b and two outwardly curved sidewalls 47 c, 47 d that connect the parallel longitudinal sidewalls 47 a, 47 b, respectively. A total length from the outer end of one curved sidewall 47 c to the opposite curved sidewall 47 b is greater than the outer diameter of the third sleeve 4 at the position of the transverse slots 46 a, 46 b, so that a portion of the pusher 47 protrudes outward when the pusher is inserted into the slots 46 a, 46 b of the third sleeve 4. The inside of the pusher is hollow to accommodate the upper portion of the first sleeve 1 therein. More in detail, the pusher 47 has a first inner portion 146 with an inner contour that matches an outer contour of the upper portion of the first sleeve 1 and adjacent thereto a second inner portion 147 with an inner contour greater than an outer contour of the first sleeve 1.

At an inner side of the curved outer sidewall 47 c a recess 148 for accommodating a spring 249 is provided. Furthermore, on one of the substantially flat long sides 47 a, 47 b, preferably near the other curved sidewall 47 d, an elongate hole 149 is provided that is elongate in the lengthwise direction in which the pusher 47 can be moved. A securing element 149 a, such as a headless screw, extends through the wall of the third sleeve 4 into the elongate recess 149 and limits the path of movement of the pusher 47 relative to the third sleeve 4 and prevents loss of the pusher 47.

The spring 249, that may be a helical spring as shown or any other kind of spring, is positioned in the recess 148 of the pusher 47 and extends into a counter-sink 49 a in the abutment plate 49.

The spring 249 is biased such, that it holds the pusher 47 in a position in which one curved sidewall 47 c protrudes outward from the transverse slot 46 a and the opposite curved sidewall 47 d is within the opposite slot 46 b. In this position, the portion 147 that has the larger inner diameter of the pusher 47 is partially narrowed by the third sleeve 4, as can be seen in FIG. 21. Hence, in this position, the first sleeve 1 fits into the portion 44 of the third sleeve 4 and is held via the pusher 47 by means of friction, as can be also seen in FIG. 4. To remove the third sleeve 4 from the first sleeve 1, the pusher 47 that protrudes outward from the slot 46 a, is pushed against the spring force into the slot 46 a so that the opposite curved outer wall 47 d protrades from the opposite slot 46 b. By means of this, the inner section 147 that has a larger diameter than the upper portion of the first sleeve 1 is brought into a position around the first sleeve 1. In this position, the third sleeve 4 can be removed by pulling the third sleeve 4 from the first sleeve 1.

The mounting of the third sleeve 4 onto the first sleeve 1 is carried out in a similar manner by pushing the pusher 47 into the slot 46 a and placing the third sleeve 4 out the first sleeve.

The parts of the extension device are all made of a body-compatible material, such as titanium or stainless steel, a body-compatible metal alloy, for example Ti-Ni-alloy, such as Nitinol, or a body-compatible plastic material, such as PEEK. The parts may be all of the same or of different materials.

Referring now to FIGS. 18 to 25 b, the attachment of the extension device to a bone anchor will be described. As shown in FIG. 23 a, the extension device is moved with the front end 1 a of the first sleeve 1 toward the receiving part 200. Then, the legs 18 a, 18 b of the first sleeve 1 are spread to a certain extent when they touch the top end 200 a of the receiving part 200 and by further downward movement of the extension device, the circumferential projections 120 a, 120 b of the legs 18 a, 18 b snap into the circumferential grooves 206 a, 206 b. Also, the vertical ribs 112 a, 112 b engage the corresponding vertical grooves 212 a, 212 b of the receiving part 200. During coupling of the first sleeve 1, the second sleeve 2 is in a retracted position, in which the collet portion 35 extends into the first portion 11 of the first sleeve 1. This configuration is shown in FIGS. 23 a to 24 b. The thickened upper edge 37 of the collet portion 35 protrudes out of the second portion 12 of the first sleeve so that an inner diameter of the collet portion 35 is at least a diameter >D₁ corresponding to an inner diameter of the threaded portion 33 of the interlocking bushing 3.

Then, as shown in FIGS. 25 a and 25 b, the second sleeve 2 is moved relative to the first sleeve 1 towards the receiving part 200 by screwing the interlocking bushing 3 further towards the front end 1 a of the first sleeve 1. When the projection 27 a, 27 b of the second sleeve 2 enter the corresponding recesses 207 a, 207 b at the top end 200 a of the receiving part 200, a rotational movement of the second sleeve 2 relative to the first sleeve 1 and therefore also relative to the receiving part 200 is inhibited. At the same time, the tapered upper thickened portion 37 of the collet portion 35 slides along the shoulder formed by the transition between the first portion 11 and the second portion 12 of the first sleeve 1 until it enters the second portion 12. Thereby, the collet portion 35 is radially compressed as shown in FIG. 25 b. An upper inner diameter of the collet portion is now D₂. The second sleeve 2 can rotate relative to the bushing 3, so that the alignment between the legs 18 a, 18 b of the first sleeve 1 and the legs 25 a, 25 b of the second sleeve 2 is maintained. A further rotation of the interlocking bushing 3 presses the flat surface portions 26 a, 26 b of the front end 2 a of the second sleeve 2 onto the free flat end surfaces of the receiving part 200. Thereby and by the engagement of the projections 120 a, 120 b, 112 a, 112 b of the first sleeve 1 in the grooves 206 a, 206 b, 212 a, 212 b of the second sleeve 2 the first sleeve 1 is interlocked with the receiving part 200 and with the second sleeve 2 to provide a safe and strong connection between the extension device and the receiving part. In such a configuration, the insertion of the rod and the locking screw can take place as well as surgical steps thereafter such as compression and distraction steps using the extension device.

The decoupling instrument that comprises an outer diameter D₁ cannot be inserted in the configuration shown in FIG. 25 b. Hence, the collet portion 35 acts as a locking member that prevents the decoupling of the extension device when the second sleeve 2 is coupled to the receiving part. This enhances the safety of the procedure and prevents damage of portions of the extension device, in particular of the inner sleeve 2.

Turning the interlocking bushing 3 in the opposite direction, releases the interlocking connection and permits to retract the projections 27 a, 27 b of the second sleeve out of the recesses 207 a, 207 b.

In the clinical use, as shown in FIG. 26, the extension device is attached to a receiving part 200 of a bone anchor. The whole bone anchor is inserted through a minimally invasive procedure into a pedicle of a vertebra. Exemplary, three vertebrae 501, 502, 503 are shown with bone anchors and mounted extension devices. By rotating the extension devices with the aid of an instrument, the channels 204 of the receiving parts of the bone anchors can be aligned to /permit the insertion of a rod. Because the connection between the receiving part 200 and the extension device is robust and safe, an easy alignment using the extension devices is possible. Thereafter, the rod is inserted through the slits 17 a, 17 b of the extension devices (not shown) and fixed with a locking screw that is guided through the extension device until it can be screwed between the legs 205 a, 205 b of the receiving part. By applying an instrument to the extension device, a compression or distraction procedure can be performed using a minimally invasive technique. Using only the first sleeve 1 and the second sleeve 2 without the third sleeve 4, has the advantage that the extension devices have a greater distance with respect to each other and that a greater angle between one extension device and another extension device can be achieved. The third sleeve 4 may be used in cases in which the surgeon has to approximate the rod.

Modifications of the above described embodiment may be contemplated. It shall be noted, that the shape of the engaging complementary structures of the first sleeve and the receiving part as well as the second sleeve and the receiving part can be modified and are not limited to the exact shape shown in the embodiments. In a further modification, the second sleeve may be coupled to the receiving part by only a frictional engagement of a portion of the second sleeve and a portion of the receiving part. For example, a front end surface of the second sleeve may be pressed against a free end surface of the legs 205 a, 205 b of the receiving part. In a further modification the first sleeve may be coupled to the receiving part by such a friction-fit.

The function of the first and the second sleeve may be interchanged. In such a case, the locking member inhibits the decoupling of the second sleeve when the first sleeve is still coupled to the receiving part.

The extension device can be used with any bone anchor that comprises a receiving part, such as a polyaxial bone anchor, a monoaxial bone anchor and can also be used with different shapes of receiving parts. Anchors with inner compression members or outer rings may be used. The only necessity is an engagement structure at the receiving part that can cooperate with a corresponding engagement structure of the extension device.

The locking member in the form of a collet portion is shown as a monolithic part of the interlocking bushing. However, a locking member can also be a separate member, for example a separate collet. Other kinds of locking members that can be actuated by actuating the second sleeve, may also be conceivable. 

1. An extension device for a bone anchor, wherein the bone anchor comprises an anchoring section for anchoring in a bone and a receiving part connected to the anchoring section, the receiving part comprising a central axis and a channel for receiving a rod, wherein sidewalls of the channel form two free legs, the extension device comprising a first sleeve with a first sleeve axis that is coaxial to the central axis, wherein the first sleeve is configured to be coupled to the receiving part and decoupled from the receiving part; a second sleeve with a second sleeve axis coaxial to the central axis, wherein the second sleeve is positioned within the first sleeve and positionable relative to the first sleeve along the central axis and wherein the second sleeve is configured to be coupled to the receiving part and decoupled from the receiving part; a locking member that in a first configuration inhibits decoupling of one of the first sleeve or the second sleeve from the receiving part when the other one of the first sleeve or the second sleeve is still coupled to the receiving part.
 2. The extension device of claim 1, wherein in a second configuration the locking member permits decoupling of one of the first sleeve or the second sleeve from the receiving part when the other one of the first sleeve or the second sleeve is already decoupled from the receiving part.
 3. The extension device of claim 1, wherein the locking member inhibits decoupling of the first sleeve when the second sleeve is coupled to the receiving part.
 4. The extension device of claim 1, wherein the locking member is sized and arranged so as to inhibit attachment of a decoupling instrument when the locking member is in the first configuration.
 5. The extension device of claim 1, wherein the locking member comprises a spring that is compressible and/or extendible in a radial direction relative to the central axis.
 6. The extension device of claim 1, wherein the locking member is formed as a collet that is positioned at least partially inside the second sleeve and that has a first inner diameter that is smaller than an outer diameter of a portion of a decoupling instrument when the locking member is in the first configuration.
 7. The extension device of claim 1, wherein the second sleeve is connected to the first sleeve through a coupling member that is configured to advance together with the second sleeve in an axial direction relative to the first sleeve.
 8. The extension device of claim 7, wherein the coupling member is a bushing that is coupled to the first sleeve through an advancement structure that permits to advance the coupling member with the second sleeve to a given position.
 9. The extension device of claim 7, wherein the coupling member is coupled to the second sleeve so that it can rotate with respect to the second sleeve.
 10. The extension device of claim 7, wherein the locking member and the coupling member are formed as a single monolithic part.
 11. The extension device of claim 1, further comprising a third sleeve that is selectively connectable to the first sleeve to enlarge an axial length of the extension device.
 12. An extension device for a bone anchor, wherein the bone anchor comprises an anchoring section for anchoring in a bone and a receiving part connected to the anchoring section, the receiving part comprising a central axis and a channel for receiving a rod, wherein sidewalls of the channel form two free legs, the extension device comprising a first sleeve with a first sleeve axis that is coaxial to the central axis, wherein the first sleeve is configured to be coupled to the receiving part and decoupled from the receiving part; a second sleeve with a second sleeve axis coaxial to the central axis, wherein the second sleeve is positioned within the first sleeve and positionable relative to the first sleeve along the central axis and wherein the second sleeve is configured to be coupled to the receiving part and decoupled from the receiving part; a third sleeve that is selectively connectable to the first sleeve to enlarge an axial length of the extension device.
 13. The extension device of claim 12 wherein the third sleeve comprises two opposite slots and a pusher extending through the slots wherein the pusher comprises an inner hollow section and wherein the first sleeve of the extension device can be clamped inside the third sleeve in a first position of the pusher and can be released in a second position of the pusher.
 14. A system of an extension device according to claim 12 and a bone anchor, wherein the bone anchor comprises the anchoring section for anchoring in a bone and the receiving part connected to the anchoring section, the receiving part comprising a central axis and a channel for receiving a rod, wherein sidewalls of the channel form two free legs and wherein the receiving part comprises an engagement structure that can be coupled to a substantially complementary engagement structure of the extension device.
 15. The system of claim 14, wherein the both, the first sleeve and the second sleeve comprise an engagement structure for a form-fit engagement with a corresponding engagement structure of the receiving part.
 16. The system of claim 14, wherein the first sleeve comprises an engagement structure for a form-fit engagement with a corresponding engagement structure of the receiving part and wherein the second sleeve is configured to be coupled to the receiving part via a frictional engagement.
 17. A spinal stabilization system for use with minimally invasive surgery comprising a system of claim 14 with at least two bone anchors and the extension device for each bone anchor. 